What is the significance of "Apiryon"?
Rashi: It is the Ohel Mo'ed, which was fixed in Mishkan Shilo. He made for it a canopy crown, for honor.
Seforno (6, 9): (Why did they need weapons in the days of Shlomo?) The crown returned to its initial state (like before the Meraglim) via building Bayis Rishon!
Malbim (Melitzah): Ha'Melech Shlomo made an Apiryon for this beloved. He wanted to make the Divine Nefesh the monarch; it should not be a concubine to the body's desires and pleasures, and submit to its command. Rather, it should be like a bride in the canopy, and like a woman who rules in the house and a queen who rules. She commands Bnos Yerushalayim 1 (bodily powers) and the king (the Yetzer and arousal) to come to her honored abode (the Mikdash that Shlomo built). Its rooms, halls and divisions illustrate the three parts of the Nefesh tied to physicality.
Malbim (Mashal): This answers how Bnos Yerushalayim let her flee to her beloved, and after she returned, they let her beloved be isolated with her. This is because ha'Melech Shlomo made an Apiryon. It was normal to seat the Kalah in a canopy on the day of the Chupah. The king wanted to betroth his beloved, to be his Kalah and Arusah; until now she was like a concubine.
Malbim: This comes to answer how Bnos Yerushalayim distanced from the beloved, i.e. how physical powers separated from his Nefesh and allowed her to be isolated with Elokim, her Beloved. Further, after the vision, his Neshamah continued to cling to Him and see with Ru'ach ha'Kodesh for many days (until his old age, when his physical wives veered his heart and he built Bamos to physical desires).
Why does it say "ha'Melech Shlomo"?
Malbim (9-10, Melitzah): "Ha'Melech Shlomo" (the body and its powers) come to the canopy of the Nefesh and betroth her with Tzedek and Avodas Hashem; she will be his wife, to father via her seed blessed to Hashem. All bodily powers will be drawn after the beloved Bas Shamayim who rules over bodily actions. Ha'Melech Shlomo hints to Shalom of the body with the Nefesh and its Yetzer; the Yetzer ha'Ra and its powers will submit to the rule of the Nefesh. Due to her, they will serve Melech ha'Kavod, who dwells with her in the canopy that he made for her.
Why does it mention "Arzei ha'Levanon"?
Seforno: Bayis Rishon was covered with cedars.
Malbim (9-10, Melitzah): All actions in the Mikdash raise physical powers to Elokim. Physical bodies offer living beings as Korbanos, Menachos and Nesachim from vegetation, and salt from Domem (lifeless). They are elevated via supreme fire, which separates the proper part. These tie with the verbal Nefesh; the Ba'al Korban intends to offer his Nefesh to be totally consumed. The body elevates to become spiritual, and makes the practical intellect act for Divine matters. Via this, also physicality (ha'Melech Shlomo and his deeds) will be intellectual and Divine. Therefore, they chose for the encompassing Nefashos of Yisrael, who dwell in the Mikdash and became a chariot for Shechinah, via physical bodies stronger than the substance of man. Cedar wood is the strongest vegetation, stronger than living beings and less composite than living bodies, like I explained above (refer to 1:17:1:3).
Malbim (Mashal): The canopy itself was of cedar wood.